Method of forming underground communication between boreholes



Sept. 13, 1960 c. A. BAYS METHOD OF FORMING UNDERGROUND COMMUNICATIONBETWEEN BOREHOLES Filed Feb. 1, 1957 "NVENTOR Carl c B B @515 ,f AGENTUnited States PatentO METHOD OF FORMING UNDERGROUND COM- MUNICATIONBETWEEN 'BOREHOLES Filed Feb. 1, 1957', Ser. No. 637,684 5 Claims.cram-3 'This invention relates to a method'of opening solid undergroundformations containing'soluble or liquescent constituents therein forfluid mining. Examples of materials which may be mined by this processare salt, potash, trona, sulfur deposits and the like.

The oil industry has employed certain processes or techniques forincreasing the porosity and permeability of an underground formation.Generally stated, these techniques involve the process of shooting orhydraulically fracturing the formation adjacent an oil producingborehole to bring about increased flow of oil. These techniques areapplicable in porous and permeable formations, such as oil bearingsands, and where the fracturing is to extend into the formation for ashort distance. Furthermore, in fracturing an oil field adjacent an oilwell it is not required that the line of fracture be substantiallycontinuous in the same horizontal'plane. Likewise, a method forrecovering soluble or liquescent deposits, such as set forth above,involves hydraulically fracturing the underground formaiton so as tobring about an underground communication between two or more boreholes.The terms borehole or boreholes when used in this applicaiton referbroadly to any borehole or point of entry into an underground formation.For example, afracture may be between a borehole and an outcrop as wellas between wells or drill holes. r

Thus, in mining a substantially non-permeable underground formationcontaining deposits such as salt, potash, trona, sulfur and likemateri-als,'it is desirable to open an underground passage between twoor more adjacent boreholes to permit the flow of liquid therebetween.The greater the distance betweenithese boreholes the more technicallyand economically feasible the mining operation becomes, as the cost ofdrilling the boreholes is many times that of hydraulically fracturingthe formation. From a technical standpoint, the longer the tures fromthe first two boreholes.

connecting two boreholes when the line of fracture from one of theboreholes has diverged in a direction away from the other borehole.

Other objects will appear to those skilled in the art as the descriptionof this invention unfolds.

Generally stated, this invention provides for the creation of anunderground interconnection between two boreholes by hydraulicallyfracturing the formation adjacent each of the boreholes. The length anddirection of the fracture from any borehole will vary and be affected bythe type formation and the pressure thereon, as well as the depth of theformation at the point of fracturing. However, in porous deposits suchas oil bearing sands as described above, the fracture length is limitedby the porosity of the deposit to about 200 feet as an average.Therefore, communication between holes more distant than 200 feet cannotbe brought about by the teachings of the prior art.

However, in solution or thermal mining between two or more boreholes ina solid, substantial non-porous underground formation, such as a tronaor salt formation, it is desirable to space the boreholes as far apartas possible and preferably at such a distance apart that the area of thefracture initially produced adjacent one Well, will not extend to theother Well. If it is desired to operate with only two boreholes, thewells should, how ever, be located sufficiently close together that theline of fracture from each of the boreholes will intercommunicate.

I have discovered that it is not of absolute necessity that the maximumfracturing distance between boreholes be predetermined in order to givethe maximum distance between boreholes and still insure that thefractures from each borehole will interco-mmunica-te. For .a third, ormore if necessary, borehole may be drilled intermediate the twoboreholes and the formation adjacent this third borehole fractured so asto communicate with the frac- By plugging or seal ing the intermediateborehole, a communication between the first two boreholes isestablished.

Even, however, where the second borehole is within the extent of theline of fracture fromthe first borehole this invention may be used tocreate an underground com-' munication between boreholes with greaterfacility. The fracture from the first borehole becomes narrow and ta-.

7 pers to a feather edge as it progresses farther away from retentiontime of the fluid within the underground cavity, r

the more efficient is the mining operation.

Heretofore, when boreholes were placed substantial distances apart, itwas impossible to communicate be tween the holes by hydraulic fracturingfrom one of the boreholes.

Furthermore, even when a borehole is disposed so as to normally bewithin the line of fracture extending from an adjacent borehole, naturalfaults, cracks and cleavages may result in a divergence of the directionof fracture away from the selected portion of the borehole it is soughtto interconnect.

. An object of this invention is to provide a method for the fluidmining of an underground deposit.

Another object isto provide a method for creating an undergroundconnection between boreholes which are spaced a substantial distancefrom each other. I A further object is to provide a -means forinterconnecting boreholes so. that a fluid may bellowed betweenboreholes from a preselected horizontal plane to another preselectedhorizontal plane.

" 1A stillfurther object is to prov idea means for interthe boreholebeing fractured. This presents frictional restraint to the flow ofliquid through the underground channel.

By fracturing from the second borehole the thickness of the fracturedzone is increased, said increase allowing the flow of liquid betweenboreholes with substantially less resistance.

Furthermore, due to the presence of natural crevices, joints andfractures in the formation, the line of fracture may deviate in a pathaway from the second borehole and only by fracturing from both boreholesmay the formation 'be solution or thermally mined at a preselectedhorizontal plane for the entrance and exit of the mining liquid to andfrom the formation.

I have further discovered that if, when fracturing from the secondborehole, to intercommunicate with the fracture from the first borehole,a substantial pressure is maintained along the line of fracture from thefirst borehole, communication between the boreholes is established withgreater facility.

In general, the process of hydraulic fracturing an underground formationcomprises drilling a borehole into theformation to the desired depth,sealing oif the bottom of the borehole if the bottom lies in relativelypermeable or porous formation, perforating the formation in the,

area desired to be fractured and pumping a fluid into the borehole togenerate sufiicient pressure on the formation at the area of perforationto cause a parting of the formation. The well may or may not be cased,but generally is cased and the casing perforated or terminated at thedepth where the fracture is to be initiated.

' 'The pressure required to part the formation is dependent on the typeof formation, but it has been found that a pressure in p.s.i.g. of from1 to 1.8 times the depth of the formation in feet to the point offracture is sufiicient. The fracture will generally form more or less ofa circular pat-tern about the borehole with the zone of fracture beingof greatest thickness adjacent the borehole and tapering off to afeather edge along the outer circumference of the circle.

As stated previously, the fracture may not extend in the same horizontalplane due to the presence of natural faults or irregularities in theformation, so that a fracture adjacent the bottom of one borehole mayintersect the second borehole several feet above or below the desiredfracture point of the other borehole.

'While the process of this invention will be described with reference tothe drilling of two boreholes and the opening of a passage therebetween,it is to be understood that the process is equally applicable tosituations where it is desired to drill a plurality of boreholes intothe formation and establish underground connections therebetween.

For purposes of illustrating an embodiment of this invention, referencewill hereafter be made to the recovery of-an underground trona depositfound in Green River, Wyoming.

Fig. 1 is a diagrammatic illustration in cross section through an earthformation showing intercomrnunication between boreholes placedsubstantial distances apart which have been hydraulically fracturedaccording to the teachings of this invention.

Fig. 2 is a diagrammatic illustration showing how interconnectionbetween boreholes may be brought about when the line of fracture fromone borehole deviates from the horizontal.

In recovering trona as shown in Fig. '1 according to the methods of thisinvention, wells A and B are drilled into the formation to a depthpreferably just beyond the lower interface of the trona bed. 'The wellsare spaced a substantial distance apart, for example, a distance of 400feet or more. The wells are cased and cemented according to conventionalpractices. After the wells are completed, the formation is perforated byconventional means at the lower interface of'the trona bed. If theformation at the bottom of the well 'is permeable, it may be necessaryto seal off the bottom of the well before fracturing. After sealing, ifnecessary, and perforation, the trona bed adjacent well A is fracturedby subjecting the perforated formation to a pressure suficient to partthe formation. The initial parting of the formation is marked by asudden drop in pressure. As shown in Fig. 1, C represents the fractureadjacent well A. I

Due to the distance between wells, a communication cannot be made byfracturing from well A alone. The next step is to similarly fracturefrom well B to create fracture D which is shown in Fig. l, ascommunicating with fracture C. Having performed the dual fracture,communication between wells A and .B is established and the tronaformation is now ready for solution mining.

If during the fracturing from well B a substantial pressure ismaintained along the line of fracture from well A to keep the fractureopen, the interconnection of the fractures is brought about with greaterfacility. This operation may be performed by holding a substantialpressure on well A, after fracturing well A, to keep the formationthrough which the fracture has penetrated from closing while fracturingfrom well B. If desired, the pressure may be relieved on well A and thenregenerated before fracturing from well B but this is usuallyundesirable as it ends to cause breaking up of the formation and the pro4 duction of innumerable small fractures instead of preserving theoriginal wider fracture from well A. Fig. 2 illustrates the situationwhich prevails when the fracture from a well deviates from the desiredhorizontal plane. By following the teachings of this invention andfracturing from the second well B, after having fractured from well A,interconnection of the fractures E and F will result and therebyestablish communication between the wells. Likewise as described inconnection with Fig. ly the interconnection of the two fractures may bebrought about with greater facility by maintaining pressure on well A tokeep the fracture E open while fracturing from well B. This will occureven if fracture F I should also follow the samefault along whichfracture E angled. As shown in Fig. 2, fracture E from well A is angledupwardly from trona bed and is extended into the overlying shaleformation and intersected well B at the point E which may be severalfeet above the trona bed and at a point where the bore of well B issealed by casing and cementing.

Example The following is an account. of a :tronarecovery opera, tion atGreen River, Wyoming, employing the teachings of this invention. Thewells A and B were drilled to a depth of 1780 feet and 1581 feetrespectively, at :which depth both wells had penetrated the tronaformation. The wells were placed at a distance of 400 feet apart.

The formation adjacent the lower interface of the trona bed at each wellwas perforated .for a distance of 3 'feet from the interface intothelower shale deposit. Well A was drilled to given borehole of 8%inches inside diameter. .A 7 inch outside diameter casing was run intothe well and cemented in. A section of Well lying :adjacent the lowerportion of the trona formation was then perforated for a distance ofabout 3 feet by :gun perforating or otherwise .rasis well known in .theoil industry. Likewise, well B was drilled to .give a borehole of 12%inch diameter. A casingof 8.6tinches diameter was run in and the wellcemented. Pressure was applied to. the perforated formation adjacentwell A by pumping water into the well. The pressure was built "up toabout 1600 p.s.i.g. in about one :rninutes time. This pressure wasmaintained for a few minutes-and resulted in the parting or fracturingof theformation as was evidenced byza pressure drop to 1020 p.-s'.i;g.However, the formationlbrealo down did not result in communication withwell B, .as was evidenced by the fact that :there was no flow ofliquidfrom well B, even though water was continually pumped into well .A.Subsequently, wellIA was sealed to hold the pressure on :the formationat about 9.25 p.s.i.'g. and water was pumpe'd-down-into well B so as togenerate a pressure of 1500 p.'s.i:g. on the formation adjacent theperforation at the bottom of wellB. .After a few minutes of thispressure, there was a parting of the formation adjacent well B :asshownby a drop in well B s pressure to 950 p.s.i.g. Water was thencontinuously pumped down well .3, the .head of well A was opened, and atrona rich liquor recovered ifl'O'l'Il well .A demonstrating theestablishments of a communication between the wells. The flow from wellA increased over a period of several days to a point where the outflowfrom well .A substantially equalled the input into well B.

By proceeding :in the -above 'manner with the additional caution ofmaintaininga pressure of about 900 psig along the line of fracture fromwell A while fracturing from well B, the channel or passage betweenwells may be enlarged in a shorter period of time after the break downof the formation adjacent well B and thus allow for greater flow fromthe output well.

While for pin-poses of "illustration the fractures C, D,' E and F havebeen illustrated as spreading substantially horizontally fromfthebot't'om's 'of wells A and B it is to be understood that the actualcourse a fracture takes is un-1 known but that wells can be connectedover substantial distances through a dense underground formation by theprocedure herein described.

Pursuant to the requirements of the patent statutes, the principle ofthis invention has been explained and exemplified in a manner so that itcan be readily practiced by those skilled in the art, suchexemplification including what is considered to represent the bestembodiment of the invention. However, it should be clearly understoodthat, within the scope of the appended claims, the invention may bepracticed by those skilled in the art, and having the benefit of thisdisclosure, otherwise than as specifically described and exemplifiedherein.

That which is claimed as patentably novel is:

1. A method for forming an underground communication between boreholesin a substantially non-permeable underground formation comprising:drilling boreholes into the formation, perforating a selected portion ofthe formation adjacent each of the boreholes, pumping a liquid into theboreholes, applying a hydraulic pressure on the liquid and the selectedportion of the formation adjacent one of the Wells to fracture theformation, maintaining a substantial hydraulic pressure along the lineof fracture from said well to keep said fracture open, then applying ahydraulic pressure on the liquid in and on the selected portion of theformation adjacent the second well sufficient to fracture the formationand bring about communication between the boreholes.

2. The method of opening a liquescent bed to solution mining, said bedlying in a substantially non-permeable underground formation, whichcomprises sinking two wells into said formation, casing and cementingsaid wells and forming openings in said wells adjacent the bottom ofsaid liquescent bed, applying a hydraulic fracturing pressure to one ofsaid wells to fracture the formation adjacent said well and extend thefracture therefrom, sealing said well to maintain the formationseparating pressure therein and keep said fracture open and applying ahydraulic fracturing pressure to the other of said wells to fracture theformation adjacent the other of said wells, causing said fractures tocommunicate and unsealing said first well to permit liquid pumped intoone of said wells to flow from the other well and remove liquescentmaterial from the formation thereby.

3. The method of opening a trona bed to solution mining, said bed lyingin a substantially non-permeable underground formation, which comprisessinking two wells into said trona formation, casing and cementing saidwells and forming openings in said wells adjacent the bottom of saidtrona 'bed, applying a hydraulic fracturing pressure to one of saidwells to fracture the formation adjacentt said well and extend thefracture therefrom, sealing said well to maintain the formation pressuretherein and keep said fnacture open and applying a hydraulic fracturingpressure to the other of said wells to fracture the formation adjacentthe other of said wells, causing said fractures to communicate andopening the seal on said first well to permit liquid pumped into the onewell to flow from the other well and remove dissolved trona from theformation thereby.

4. The method of opening a soluble salt bed lying above an insolublelayer in a dense non-porous formation, to solution mining, whichcomprises the steps of drilling two wells into the formationsubstantially to the bottom of the soluble salt formation, injectingliquid into one of said Wells under sufiicient pressure to effect astratum fracture adjacent the bottom of said well, maintaining a liquidpressure on said well sufiicient to maintain said fracture open,injecting liquid into the other of said wells to open a passage betweensaid wells and releasing the pressure on one of said wells Whilecontinuing to pump a. dissolution liquid into the other well to dissolveout a passage between said wells.

5. The method of mining a soluble mineral frdm a stratum betweeninjection wells and production wells, said method comprising the stepsof pumping a fracturing fluid into one of said Wells under suflicientpressure to produce a fracture in the formation at the base of saidwell, maintaining the fluid in said well under suflicient pressure tokeep said fracture open, pumping a fracturing fluid into another of saidwells under sufiicient pressure to produce a fracture at the base ofsaid other well and continuing to pump fluid into said well undersutficient pressure to keep said fracture open while maintaining saidfirst well under sufiicient pressure to keep the fnacture around saidfirst well open until the said frac tures are interconnected and thenrelieving the fracture, maintaining pressure on one of said wells whilecontinuing to pump a dissolving fluid for said mineral into the other ofsaid wells and through the formation between said wells and out of theother of said wells to dissolve mineral from the [formation along thepassage thus opened.

References Cited in the file of this patent UNITED STATES PATENTS1,960,932 Tracy May 29, 1934 2,584,605 Merriam et al Feb. 5, 19522,642,142 Clark June 16, 1953 2,758,653 Desbrow Aug. 14, 1956 2,780,449Fisher et al Feb. 5, 1957 2,780,450 Ljungstrom Feb. 5, 1957 2,847,202Pullen Jr Aug. 12, 1958 2,850,270 Hanson Sept. 2, 1958 FOREIGN PATENTS716,620 Great Britain Oct. 31, 1954

